I've long stopped doing Time Machine backups due to it taking what seemed to be forever to do. Why it's slow, I do not know. Granted, I am still using the Time Capsule that I bought in 2009, but I only use it for Wi-Fi purposes and as an external drive.

Yesterday the 2015 Mac stated 18 hours to go, now it states 57 minutes remain at 11 MBps via hard-wired ethernet but it's definitely not going down. The Mac isn't frozen though and I feel the hard drive reading but not super rapidly. Is there any advanced view function of the transfer to see where it's at? A simple blue progress bar and minutes isn't very promising.

I backed up my late 2011 iMac a few days ago with a Western Digital Passport drive that is USB 3 compatible. It took about ~14 hours to backup, so I did it overnight and it was done the next day.

I purchased the new late 2015 iMac with 4Ghz i7 and 3TB Fusion Drive, and using the same drive and USB connection, it's estimating the time to fully restore the new iMac is a bit over 2 hours. It's been going for about 20 minutes or so, and only has an hour and 47 minutes left. I'm getting an average transfer rate for about 103MB/s, which is pretty good.

I'm not sure I would do a full restore off a network drive unless I absolutely had to. Too many bottlenecks.